A person's circulatory system includes both systemic and pulmonary circulation systems. Pulmonary circulation supplies the lungs with blood flow, while the systemic circulation takes care of all the other parts of the body, i.e. the systemic circulation. The heart serves as a pump that keeps up the circulation of the blood. Both the pulmonary and systemic circulatory systems are made up of arteries, arterioles, capillaries, venules and veins. The arteries take the blood from the heart, while the veins return the blood to the heart
Blood pressure is defined as the force exerted by the blood against any unit area of the vessel wall. The measurement unit of blood pressure is millimeters of mercury (mmHg). Pulmonary and systemic arterial pressures are pulsatile, having systolic and diastolic pressure values. The highest recorded pressure reading is called systolic pressure, which results from the active contraction of the ventricle. Although the arterial pressure and indeed flow in the arteries is pulsatile, the total volume of blood in the circulation remains constant. The lowest pressure reading is called diastolic pressure which is maintained by the resistance created by the smaller blood vessels still on the arterial side of the circulatory system (arterioles). Stated another way, the systolic pressure is defined as the peak pressure in the arteries, which occurs near the beginning of a cardiac cycle. In contrast, the diastolic pressure is the lowest pressure, which occurs at the resting phase of the cardiac cycle. The pulse pressure reflects the difference between the maximum and minimum pressures measured (i.e., the difference between the systolic pressure and diastolic pressure). The mean arterial pressure is the average pressure throughout the cardiac cycle.
Arterial pulse pressure, such as mean arterial pressure (MAP), is a fundamental clinical parameter used in the assessment of hemodynamic status of a patient. Mean arterial pressure can be estimated from real pressure data in a variety of ways. Among the techniques that have been proposed, two are presented below. In these formulas, SP is the systolic blood pressure, and DP is diastolic pressure.
                    a        .                                  ⁢        M            ⁢                          ⁢      A      ⁢                          ⁢              P        2              =                            (                                    S              ⁢                                                          ⁢              P                        +                          2              ⁢                                                          ⁢              D              ⁢                                                          ⁢              P                                )                /        3            =                                    1            3                    ⁢                      (                          S              ⁢                                                          ⁢              P                        )                          +                              2            3                    ⁢                      (                          D              ⁢                                                          ⁢              P                        )                                                  b        .                                  ⁢        M            ⁢                          ⁢      A      ⁢                          ⁢              P        1              =                  (                              S            ⁢                                                  ⁢            P                    +                      D            ⁢                                                  ⁢            P                          )            /      2      
Systolic pressure and diastolic pressure can be obtained in a number of ways. A common approach is to use a stethoscope, an occlusive cuff, and a pressure manometer. However, such an approach is slow, requires the intervention of a skilled clinician and does not provide timely readings as it is a measurement at only a single point in time. While systolic pressure and diastolic pressure can also be obtained in more automated fashions, it is not always practical to obtain measures of pressure using a cuff and pressure transducer combination, especially if the intention or desire is to implant a sensor that can monitor arterial pressure on a chronic basis.
Another approach for obtaining measures of arterial pressure is to use an intravascular pressure transducer. However, an intravascular device may cause problems, such as, embolization, nerve damage, infection, bleeding and/or vessel wall damage. Additionally, the implantation of an intravascular lead requires a highly skilled physician such as a surgeon, electrophysiologist, or interventional cardiologist.
Plethysmography, the measurement of volume of an organ or body part, has a history that extends over 100 years. Photoplethysmography (PPG) uses optical techniques to perform volume measurements, and was first described in the 1930s. While best known for their role in pulse oximetry, PPG sensors have also been used to indirectly measure blood pressure. For example, non-invasive PPG sensors have been used in combination with in an inflatable cuff in a device known as Finapres. U.S. Pat. No. 4,406,289 (Wesseling et al.) and U.S. Pat. No. 4,475,940 (Hyndman) are exemplary patents that relate to the Finapres technique. The cuff is applied to a patient's finger, and the PPG sensor measures the absorption at a wavelength specific for hemoglobin. After the cuff is used to measure the individual's mean arterial pressure, the cuff pressure around the finger is then varied to maintain the transmural pressure at zero as determined by the PPG sensor. The Finapres device tracks the intra-arterial pressure wave by adjusting the cuff pressure to maintain the optical absorption constant at all times.
There are a number of disadvantages to the Finapres technique. For example, when there exists peripheral vasoconstriction, poor vascular circulation, or other factors, the blood pressure measured in a finger is not necessarily representative of central blood pressure. Further, maintaining continuous cuff pressure causes restriction of the circulation in the finger being used, which is uncomfortable when maintained for extended periods of time. Accordingly, the Finapres technique is not practical for chronic use. Additionally, because of the need for a pneumatic cuff, a Finapres device can not be used as an implanted sensor.
Simple external blood pressure monitors also exist, but they do not offer continuous measurement and data logging capability. These devices can be purchased at a drug store, but patient compliance is required to make regular measurements and accurately record the data. Additionally, portable external miniature monitors that automatically log blood pressure data exist, but these devices can only store a day or so of data and require clinician interaction to download and process the measured data.
As is evident from the above description, there is the need for improved systems and methods for monitoring arterial blood pressure, including systolic pressure, diastolic pressure and mean arterial pressure.
Ventricular afterload, also known as cardiac afterload (CA), may be defined as the mechanical force opposing ventricular ejection, as for example described by W. R. Milnor, “Arterial Impedance as Ventricular Load,” Circulation Research, 1975;36:565-70. This mechanical opposition of the flow of the viscous blood through the visco-elastic arterial system has two major mechanical components determined by the mechanical properties of the arterial system including hydraulic resistance and arterial compliance.
Hydraulic resistance is a function of several factors including the smooth muscle tone of the arterial system that determines arterial dimension, the dimensions and patency of the aortic or pulmonic valve, the geometry of the ventricular outflow tract, thickness of the ventricular myocardium, the length of the arterial vessels and the viscosity of the blood. Hydraulic resistance is proportional to ventricular afterload and can be described in general by Poisuelle's law or by Ohm's law, which states that systemic vascular resistance (also referred to as total peripheral resistance) is equal to the difference between mean arterial pressure and central venous pressure divided by cardiac output. Hydraulic resistance is typically estimated clinically by invasive or non-invasive estimates of mean arterial pressure and cardiac output.
Arterial compliance describes the ability of the arterial blood vessels to store a portion of the energy delivered to the arterial system by the ventricles during systole and return that energy to the arterial blood during ventricular diastole in order to maintain diastolic arterial blood pressure and flow. Arterial compliance is inversely proportional to ventricular afterload. Clinical estimates of arterial compliance are difficult to measure. It is occasionally approximated by aortic distensibility, or the change in aortic pressure divided by the change in aortic cross-sectional area. Another estimate of arterial compliance is “effective arterial elastance” as described, for example, by R. P Kelly et al., in “Effective Arterial Elastance as an Index of Arterial Vascular Load in Humans,” Circulation 1992;86:513-521. Estimation of this parameter requires measurement of ventricular pressure and volume.
Ventricular afterload includes both arterial resistance and arterial compliance, and may also be estimated using lumped or distributed mathematical models such as for example the three-element Windkessel model described by K. H. Wesseling et al., “Computation of Aortic Flow from Pressure in Humans Using a Non-linear, Three-element Model,” J. Appl. Physiol., 1993;74:425-35. The mathematical solution to these models requires measurement of both aortic blood pressure and flow.
The term “ventricular arterial coupling” describes the mechanical relationship between the ventricles and the arterial system during ventricular ejection as described for example by M. R. Starling, “Left Ventricular-arterial Coupling Relations in the Normal Human Heart,” Am. Heart J., 1993;125:1659-66. Cardiovascular function may be maintained even if ventricular contractile function is reduced by a compensatory decrease in ventricular afterload (either by decreased resistance, increased compliance or both). For example, administration of nitroglycerin during an episode of myocardial ischemia can maintain cardiac output despite decreased ventricular contractility by reducing arterial tone, increasing arterial compliance and hence decreasing ventricular afterload. Measurement of ventricular arterial coupling parameters involves measurement of both ventricular pressure and volume.
Regional or global changes in ventricular afterload including arterial resistance and compliance may alter patterns of arterial wave reflection. These changes in arterial wave reflection patterns may be manifest by changes in pressure signals measured in the arteries or ventricles as demonstrated for example by M. O'Rourke, “Coupling Between the Left Ventricle and Arterial System in Hypertension,” Eur. Heart J. 1990;11(G):24-28. Thus, changes in the morphometry of ventricular or arterial blood pressure signals can indicate changes in the resistive and compliant properties of the arterial system and hence can indicate changes in ventricular afterload.
The core of the altered cardiovascular function in HF is a depression of cardiac contractility. Therefore, an adequate assessment of cardiovascular function, including right or left ventricular afterload, has important diagnostic and therapeutic implications. Patients with acute HF, particularly as a complication of acute myocardial infarction or as an acute exacerbation of a previously compensated chronic HF, have a high mortality rate of about 30% within the first 12 months. In this clinical condition, a proper evaluation of ventricular afterload is extremely important for diagnostic purposes to assess the severity of the process and as a guide for the inotropic, vasodilator, or diuretic therapy. Typically, resistance indices are used to evaluate ventricular afterload, such as systolic arterial blood pressure, systemic vascular resistance or peak ventricular wall stress, with the serious limitations that these parameters have, since they ignore arterial compliance. Ventricular afterload may be estimated using aortic (or pulmonary) input impedance. However, this index requires the measurement of both pressure and flow and is difficult to interpret clinically.
Multiple clinical pathologies may result in acute or chronic changes in ventricular afterload including valvular disease, hypertension, ventricular hypertrophy, hypertrophic cardiomyopathy, atherosclerotic plaque formation, arterial thrombus, systemic shock, etc. In addition any vasoactive substance that affects arterial or venous tone, such as but not limited to nitro-glycerin, sodium nitro-prusside, neosynephrine, or epinephrine, can dramatically alter ventricular afterload. Hence, the ability to monitor ventricular afterload is extremely desirable. Further, it would be desirable if such ability was provided to a chronically implantable cardiac device.
An implantable system for measuring ventricular afterload has been proposed in U.S. Pat. No. 6,887,207. However, the proposed implantable system includes a pressure sensor implanted with the right ventricle near the outflow tract of a patient's heart. Accordingly, the proposed system relies on invasive intracardiac sensors. Such an intracardiac sensor may cause problems, such as, embolization, nerve damage, infection, bleeding and/or cardiac or vessel wall damage. Additionally, the implantation of an intracardiac sensor requires a highly skilled physician such as a surgeon, electrophysiologist, or interventional cardiologist. Accordingly, there is still a need for improved systems and methods for monitoring ventricular afterload.